1. Field of the Invention
The present invention relates to assisted movement of a flexible membrane or restraint. More particularly, the invention relates to the controlled movement of a restraint using a power-assisted actuator.
2. Background of the Related Art
Coverings or restraints are necessary or beneficial in many applications to protect underlying members. When the underlying member has a moveable component, such as a hinged lid, arm or joint, it is generally desirable for the restraint to move along with the component. Depending upon the application, the restraint itself can be hinged or sufficiently oversized to avoid restricting movement of the component. However, it is often also desirable for the restraint to be substantially conformal to the component. For example, body suits and gloves used to protect a person from natural or induced hostile environments preferably have restraint layers fashioned to fit closely or conformally to the relevant body member so that the suit does not prevent or hinder performance of a task.
When discussing the movement of a body member, it is often useful to describe the movement by reference to the jointed motion. For example, when a joint, such as the human elbow, is bent, the joint is said to be flexed and movement in that direction is referred to as flexion. When the bent joint is straightened, the joint is said to be extended and movement in that direction is referred to as extension. However, flexion and extension of the joint are accomplished by flexing muscles that are connected to opposing sides of the joint. The muscles themselves are incapable of pushing. With regard to the human body, flexion and extension applies to the knee, finger joints, finger metacarpophalangeal (MCP) joints, the waist and others. These same terms, flexion and extension, may be applied to the movement of restraints.
In the performance of many tasks, gloves are a person's primary interface with equipment and tools and must be as easy, efficient and comfortable to use as possible. It is also common for gloves to provide thermal protection from hot equipment and include high friction surfaces on the palm and finger fronts to increase dexterity. To provide these desired properties, gloves and other restraints can become thick, resilient and restrictive to the natural movement of the underlying member. Therefore, it is desirable that gloves and other restraints be kept as thin as possible to maintain tactility and reduce the amount of torque necessary to flex or extend the restraint.
It is desirable that the areas of a restraint covering a jointed member, such as the finger MCP joint areas of a glove, be both thin and conformal, because flexion of a joint causes the longitudinal length on the bent side of the joint to be reduced and excess material to gather in these areas. This gathering hinders the movement of the joint and can act as a standoff interfering with the function of the underlying member. For example, where the restraint is a glove on a hand, the gathered material can prevent objects from entering the palm fully, and thus making grasping difficult.
One particularly complex and demanding restraint is the space suit and gloves worn by an astronaut during extra vehicular activities (EVA). During an EVA, an astronaut's ability to perform efficient and accurate work outside the spacecraft is critical for the success of the mission. A critical factor that limits the ability of the astronaut to perform an EVA is the amount of torque that each joint of the body must apply to flex the heavy space suit. By moving a body member in the desired direction, the member pushes against the inside wall of the suit, which is essentially a fabric pressure vessel. Specially constructed joints in the suit move in response to the astronaut's movement, but require a certain amount of torque to operate. The torque required to move the joint is usually proportional to the internal pressure, with higher pressures resulting in higher joint torque.
Many different tasks may be required during a single EVA, depending upon the mission. These tasks may include the use of various hand tools, grasping of larger objects and operating difficult levers such as parking electrical connectors in place. It may be even be necessary to retrieve and position loose nuts onto a bolt. Although there are four finger MCP joints per hand, most pressure suit gloves group all four joints together and treat them as if they function as one. The four joints are grouped together because these joints are used primarily when grasping, during which all four fingers are flexed simultaneously.
The Extravehicular Mobility Unit (EMU), which is the space suit currently used on board the space shuttle, operates nominally at 4.3 pounds per square inch absolute (psia). At this low pressure, astronauts must breath pure oxygen for a period of almost two hours before executing an extra vehicular activity in order to avoid decompression sickness. To increase mission efficiency, space suits are currently being designed to operate at 8.3 psia, the lowest pressure which does not require astronauts to pre-breathe pure oxygen. In order to make a space suit operable at this higher pressure, metals or hard composite structures are required to make many parts. Consequently, some joints, particularly those of the glove, will become quite difficult to operate. The amount of torque required to actuate the glove is the difference between the astronaut's torque output and the torque applied to the tool or object. Gloves that require high actuation torque can cause premature fatigue of the hands and can limit mission capabilities, thereby decreasing the chance for mission success and posing an increase safety risk to the astronaut. For these reasons, low torque flexion of the glove is critical to mission success.
The performance of any restraint that covers a joint or hinge is determined by three major design characteristics: 1) the angular range of motion, 2) the torque required to move it through its complete range, and 3) stability throughout the joint's range of motion. To maintain a high angular range of motion, it is desirable to employ restraint joints that provide at least the full range of motion of the jointed member therein, such as that of a astronaut's finger. Reducing the torque required to move the restraint, conserves effort that could be used to accomplish other objectives. Joint stability is the tendency of a joint to maintain it at a fixed angular position without the application of a constant torque. A joint is said to be unstable at a position when it requires torque input to remain static. For example, fabric joints may become unstable at the extremes of their range of motion. Conversely, a joint is said to be stable when it requires no torque to remain at a fixed position. Various joint designs may be unstable at neutral points but stable at extremes or vice versa.
Flexion of restraints that are made of multiple fabric layers may require increased amounts of torque in order to overcome friction between the layers. Friction is also caused by the jointed member rubbing against inner walls of the restraint, ball bearings scrubbing their races, pressure in environmental seals within the bearings rubbing their sealing surfaces and fabric layers rubbing against adjacent components. In applications where the restraint forms a pressure vessel, compression of joint internal volume may occur during flexion, particularly at the range extremes. This compression requires work and energy to accomplish, and tends to oppose flexion.
EMU gloves, such as the 3000 series glove, require particularly high torques, because they incorporate multiple fabric layers and also are pressure vessels. Current EMU gloves employ flat pattern joints for the fingers and thumb and a hybrid joint with an external gimbal ring and restraint lines to allow omnidirectional wrist movement. The pressure bladder, a single piece made by dipping in mold in urethane solution, is made larger than the restraint so as to not encounter loading in the plane of the wall of any kind. The restraint layer incorporates joint features to allow motion. A third layer over glove provides thermal insulation, and has flat pattern joints for the fingers to allow motion.
Because fabric pressure vessels naturally try to achieve a circular shape in any cross-section and the cross-section through the hand at the palm creases should be flat, EMU glove designs incorporate special features to the glove which make the pressurized shape more conformal. The most effective device is a flattened palm bar and strap which crosses the palm just below the MCP joints of the fingers to compress the glove into an oval cross-section that is more conformal to the hand. The palm bar spans the palm side of the glove and extends roughly 70 around to the back of the glove on each side. The ends of the strap are typically coupled with an adjustable buckle to retain the bar in position. The palm bar typically has a diameter of about 0.156 inches and is made of 300 series stainless steel with flattened ends for comfort.
Because a relatively high torque is required to move the MCP joints of the glove, as compared with the tips of the fingers and thumb, astronauts frequently modify their grip by flexing their fingers and thumbs further than normal while keeping their MCP joints straight. This type of grip tends to over utilize the fingers and thumb because they flex further than normal. Flexion into this further range increases torque required to bend these joints which brings the onset of fatigue earlier than if the MCP joint were used more during grasping.
EMU glove limitations require that tools and EVA compatible equipment be furnished with grasping points that are large, approximately 2 inches in diameter, to reduce fatigue induced by grasping. Levers, switches and other devices are made large enough to allow actuation with an open hand and designed so as not to require finger/thumb opposition where possible. Furthermore, the maximum allowable force to actuate an EVA tool is also limited due, at least in part, to glove limitations. Hardware placed in the palm reduces tactility to an unacceptable level.
Therefore, there is a need for a method and apparatus for reducing the amount of torque required to actuate a restraint layer, such as a glove, covering a moveable member, such as a hand. There is also a need for an apparatus that is external to the restraint layer, thereby allowing the device to be much smaller and lighter because it does not have to meet life critical safety standards. Furthermore, it would be desirable if the apparatus did not noticeably interfere with movement of the joint, i.e. does not change the geometry of grasping in the palm or decrease the dexterity of the hand by requiring unnatural movements for operation. It would be further desirable if the joint were power assisted and incorporated a power transmission method that allowed use of a remote motor. It would also be desirable if these apparatus did not exceed stress limits on the fabric restraint layer or damage it by forcing movement in a way that induces premature failure of the restraint. It would also be desirable if the apparatus did not limit joint travel to unacceptably small angles, but rather would increase the range of some fabric or hybrid joints by applying the needed torque to compress volume at the extremes. Furthermore, in applications where the restraint forms a glove, the tactility and grasping geometry of the glove with relation to the grasped object should be maintained.